Bone prostheses are often needed for temporary or permanent use in man or animals. A wide variety of different biocompatible materials have been developed for use as bone prostheses, including, for example, natural or synthetic mineral materials, metals, such as Vitallium.TM., stainless steel and chromium alloys, as well as organic resins, such as silicone rubbers. The foregoing materials may be employed, for example to: (1) replace a portion of bone which has been lost due to accident or disease, or (2) reinforce a portion of bone which has atrophied or suffered a reduction in mineral content.
In some individuals the alveolar ridge becomes abnormally thin and unable to support either natural or artificial teeth. The support or rebuilding of the alveolar ridge has, therefore, become an important step in the treatment of those individuals suffering from a weakening in the alveolar ridge due to periodontal disease or other causes. Mineral materials of both synthetic and natural origins have been employed for bone restorative purposes in the alveolar ridge and, hence, to prevent tooth loss due to bone loss in the alveolar ridge.
Many of the same synthetic and naturally occurring biocompatible materials which have been employed for bone prosthesis have also been employed for dental restorative purposes. In particular, calcium phosphates, such as hydroxylapatite, tricalcium phosphate (whitlockite) and mixtures thereof have been widely reported in the literature as suitable for use as bone prostheses as well as for dental restorative purposes. See, e.g. Monroe et al., J.Dent.Res. 50, pp. 860 et seq. (1971); Rao & Boehm, J.Dent.Res. 53, p. 1353 et seq. (1974); Bett et al., J.A.C.S. 89, p. 5335 et seq. (1967); Kutty, Indian J.Chem. 11, 695 (1973).
Hydroxylapatite is a naturally occurring mineral present in phosphate rock. Hydroxylapatite also constitutes the mineral portion of natural bone and tooth. As such it is highly biocompatible and has a thermal coefficient of expansion quite similar to tooth enamel.
As discussed in greater detail below, in accordance with the preferred embodiments of the method of this invention, fine dry particles of a hydroxylapatite powder are agglomerated with a binding agent into sinterable spheroidal agglomerates. The spheroidal agglomerates are then sintered to provide spheroidal ceramic particles of hydroxylapatite having a uniform network of micropores extending throughout the ceramic product.
U.S. Pat. No. 4,097,935 (hereinafter '935) sets forth a description of a method for preparing a maximally densified, pore-free hydroxylapatite ceramic body. In accordance with the '935 patent the dense, pore-free ceramic body described therein may be prepared by sintering (under specified conditions) a shaped body or mass prepared from an aqueous gelatinous precipitate of hydroxylapatite. The '935 patent teaches away from the use of both products and processes which employ fine particles of hydroxylapatite as starting materials in the preparation of the dense, pore-free ceramic products described in the '935 patent. In this regard the '935 patent states:
It is critical, in the above process, to prepare the hydroxylapatite as a gelatinous precipitate from aqueous solution for it is only in this cohesive gelatinous state that hydroxylapatite can be shaped or molded and then dried and sintered to produce a ceramic body. Dry particulate or granular hydroxylapatite cannot be reconstituted into the cohesive gelatinous state . . . Moreover although powdered hydroxylapatite can be compressed into a shaped body, such as a tablet, when sintered according to the method of this invention the product obtained is highly porous and does not fracture along smooth planes but simply shatters. (Col. 9, lns. 22-39).
In contrast to the foregoing the method of this invention employs dry particulate hydroxylapatite as the starting material in a novel method employed to prepare porous hydroxylapatite ceramic particles having a network of micropores extending throughout the ceramic product.
The '935 patent also discloses means for introducing pores into the ceramic bodies produced in accordance with the method described in that patent. In this regard the '935 patent states that pores may be introduced by drilling or machining holes in the non-porous ceramic product, or by mixing an organic binder with a body of the gelatinous hydroxylapatite precipitate prior to sintering. The binder is said to volatilize during sintering to produce pores in the ceramic product. The sintered body would then have to be ground, or comminuted in some other way to provide a particulate ceramic product.
Unlike the method described in the '935 patent, in accordance with applicant's method, a binding agent is not added to a gelatinous precipitate of hydroxylapatite, and in producing applicant's final ceramic it is not a sinterable body prepared by adding a binding agent to an aqueous gel which is ultimately sintered. Rather, contrary to the method described in the '935 patent, in accordance with applicant's method the binding agent is employed to agglomerate together fine dry particles of hydroxylapatite, and it is applicant's novel agglomerate of dry hydroxylapatite particles which is sintered in accordance with the method of the present application.
Biocompatible compositions suitable for use as a dental filling material have been prepared by mixing finely divided ceramics such as sintered hydroxylapatite with a hardenable binder material. In addition, moist ceramic particles of hydroxylapatite have been employed as a biocompatible packing material to fill the voids or lesions caused by advanced periodontal diseases. The ceramic particles used have typically been employed in the form of very finely divided ceramic powders made up of particles in the range of about 10 to about 60 mesh.
Fine particles of calcium phosphate ceramics suitable for use in such filling or packing compositions may be prepared by grinding larger particles or masses of the ceramic down to fine particles within the desired particle size range. The grinding step may be conducted before or after sintering. However, in order to obtain a ceramic powder made up of particles within a desired size range, particles larger and particles smaller than desired must be separated by sieving or by another particle classification process, from the mass of particles produced by the grinding step. Thus, grinding processes typically yield a fraction of ceramic particles which are smaller than the desired particle size range, and which are often simply discarded as waste. Moreover, the ceramic particles produced by grinding are typically not uniform in shape, and possess sharp edges or "points" which could lead to local inflammation when placed in contact with tissue.
Ground hydroxylapatite particles and other ceramic particles having sharp edges or points can be mechanically treated to render the particles substantially spheroidal in shape and smooth. However, such mechanical procedures involve extensive milling to remove the sharp edges from the ceramic particles. The process itself is very cumbersome, and the yields quite low.
Conventional molding, casting or pressing operations, which do not involve grinding or milling, are generally suitable for the preparation of smooth round ceramic particles. However, in the case of calcium phosphate and other ceramics intended for use in bone or tooth restorative compositions, particles on the order of 20-40 mesh are often desired, and such particles are too small to be produced by the conventional fabrication processes known to be useful to prepare round smooth particles.
It is an objective of this invention to provide a substantially waste-free, high-yield, ceramic particle-forming process which may be employed to prepare ceramic particles which are substantially spheroidal in shape, and are within a desired particle size range.
It is a particular objective of this invention to provide a high-yield process for the preparation of biocompatible ceramic particles, especially particles of calcium phosphate and aluminum oxide ceramic which are substantially spheroidal in shape and within about the 10 to about 80 mesh range. The spheroidal ceramic particles produced by the process of this invention are free of sharp edges or ridges capable of producing local irritation when placed in contact with tissue. As such, the spheroidal ceramic particles of this invention are suitable for use as the ceramic component of hardenable binder compositions formulated for use for dental or bone restorative purposes.